- Identification of the weathering products in waste forms and the determination of the speciation ofthe pollutants in the solid and aqueous phase
Example: Antimony weathering products in mine wastes
Mining – Examples of services
Antimony is classified as a priority pollutant; it is known to be toxic and probably also carcinogenic.
Nevertheless, little attention is paid to this pollutant in comparison with its geochemical cousin, the element arsenic, because they commonly occur together. At past or active mining sites, the primary Sb minerals (mostly stibnite, Sb2S3) weather to form Sb oxides, Sb-rich hydrous ferric oxide, or to release Sb into the environment.
We have studyed the abandoned Sb mine waste impoundments in Slovakia. The focus of the work is the identification of the weathering products in these waste forms and the determination of the speciation of the main pollutants in the solid and aqueous phase. That is, the speciation of both arsenic and antimony is studied, with emphasis on the secondary Sb minerals and the competition of As and Sb for adsorption sites on other minerals.
The weathering products are mostly Fe oxides enriched in Sb and As (Fig. 1). A minor portion of the grains represents Sb oxides where Sb prevails over Fe and As. The backscattered electron images (Fig. 2) suggest that the Fe-rich grains grew by precipitation from the aqueous solutions. On the other hand, the Sb-rich grains formed most likely by in-situ oxidation and transformation from stibnite (Sb2S3).
X-ray maps from the SUL-X beamline at the ANKA synchrotron source confirmed the close association of Fe and As in the studied samples (Fig. 3). The distribution of Sb could not be probed yet. X-ray absorption near-edge structure (XANES) spectra measured on multiple weathering grains showed that arsenic is always present in its highest oxidation state, i.e., as As5+ (Fig. 4). The collected extended X-ray absorption fine-structure (EXAFS) spectra can be interpreted in terms of tetrahedral coordination of As by oxygen (as AsO4 3–); the weaker features in the second coordination shell correspond probably to the bidentate-binuclear complex that has been postulated on the iron oxides in previously published studies.
Currently, we are analysing the As EXAFS spectra from grains where Sb predominates and where Sb is assumed to be pentavalent. While the bond lengths Fe3+-O and Sb5+- O are almost identical, the charge on the two cations is different. Therefore, these results will give us insight into the competition of Sb and As for the binding sites on iron oxides and well as association of As and Sb in oxidic weathering products.